Baseline characteristics are not sufficient indicators of non-response bias in follow up studies

Abstract

Study objective - The aim was to examine whether baseline characteristics from a cross sectional survey provided sufficient information regarding non-response bias in a follow up study when compared with information on hospital admissions in the intervening years. Design - This was an 11 year follow up study of a cohort selected in 1974 with register information on hospital admissions during follow up. Setting - The study was based on a sample of cement workers from a particular Portland cement factory with suitable controls from other occupations. Participants - A total of 1404 men participated in the first survey in 1974, including a questionnaire and lung function tests. In 1985 1070 men were alive and of these, 928 men (87%) responded to a postal questionnaire. Main results - Non-responders in 1985 did not differ markedly from responders when smoking habits, respiratory symptoms, and lung function were examined in 1974. During follow up, non-responders had twice as high rates of hospital admission due to respiratory diseases as responders. These differences remained present after adjusting for minor differences in age and smoking habits. Conclusions - Equal distributions of baseline characteristics among responders and non-responders in a follow up study do not preclude non-response bias.